1. Field of the Invention
This invention relates to thermal transfer printing and more particularly, to a thermal dye transfer printing system, and also to thermal printing sheets and dye-receiving sheets which are most suitable for the transfer printing system to obtain full color hard copies of a high quality at low costs.
2. Description of the Prior Art
Several thermal transfer printing methods are known in which a half-tone image of a high quality can be obtained using an ink carrier sheet in an efficient manner. For instance, there has been proposed a method, for example, in Japanese Laid-open patent application No. 59-129196. In the method, a thermal transfer ink carrier film (which may be hereinafter referred to simply as transfer sheet) having a dye carrier layer, in which a thermally fusible ink is impregnated in a porous network structure, is fed at a speed lower than a speed of a printing paper (hereinafter referred to as dye or image acceptor sheet) on which the ink is transferred in an imagewise pattern. Another method has been also proposed, for example, in Collected Papers of the First Symposium on Non-impact Printing Techniques, "Characteristic Properties of Transfer Reaction-type Thermal Printing Papers", by Kubo et al, p. 39, 1984. In the method, a transfer sheet having a dye carrier layer mainly composed of leuco dyes is fed at a speed lower than a speed of a dye acceptor sheet which has a dye-receiving layer mainly composed of color developers capable of developing a color by coupling with the leuco dye on melting. An imagewise pattern is printed on the dye acceptor sheet by means of a thermal printing head.
Both methods described above utilize a relative speed system in which the feeding speed of the transfer sheet relative to the thermal printing head is lower than a feeding speed of the dye acceptor sheet relative to the thermal printing head, thereby ensuring effective utilization of the transfer sheet. When the feeding speed of the dye acceptor sheet is taken as v, the speed of the transfer sheet is determined as v/n in which n&gt;1. This relative speed system is advantageous over systems, in which the dye transfer sheet is repeatedly used for printing, in that because a fresh portion of the dye carrier layer is invariably in a printing zone, a variability in amount of a remaining dye in the printing zone during the printing process can be more reduced than in the latter system.
In these known methods, the dye carrier layer or both the dye carrier layer and the dye-receiving layer melt on printing, by which lubricity is imparted to the thermal transfer system. However, these methods are disadvantageous in that a picture element is apt to blur and an amount of a transferred dye does not become uniform owing to the difficulty in uniform contact between the transfer sheet and the dye acceptor sheet. As a result, a stable halt tone cannot necessarily be reproduced without formation of an image of a smooth half tone.
On the other hand, a thermal transfer printing system using sublimable dyes is known, which ensures reproduction of a stable and smooth half-tone image of a high quality. This type of transfer sheet system using sublimable dyes has been proposed, for example, in Japanese Laid-open Patent Application No. 59-88981. The transfer sheet has, on a substrate, a dye carrier layer of a dispersion of a sublimable dye and non-sublimable particles in a binder resin. However, because the non-sublimable particles are not lubricating in nature, the transfer sheet will stick with an image acceptor sheet when they are fed at different relative speeds or will bring about an excessive frictional force on contact with the image acceptor sheet, thereby causing the sheets to travel unstably and the surface of the image acceptor sheet to be damaged.
Image acceptor sheets for use in the above type of thermal transfer printing system using sublimable dyes are also described, for example, in Japanese Laid-open patent application Nos. 57-107885 and 58-148794. These image acceptor sheets include a dye carrier layer containing a saturated polyester resin with or without fine powder of silica. However, this type of image acceptor sheet is not satisfactory for printing by feeding the transfer sheet and the acceptor sheet at different relative speeds, because the acceptor sheet is relatively poor with respect to the heat resistance, lubricity and smoothness. As a result, sticking or an excess of friction takes place, thus causing the sheets not to travel smoothly or deteriorate in image quality.